Corrosion will reduce the useful life of a structure. Corrosion may result in the thinning of the structure, pitting of the structure, or cracking of the structure. The type of corrosion that may occur and the type of corrosion monitoring systems needed to assess the degree of corrosion will depend on the service environment of the structure and the condition and operational use of the structure. There are three basic approaches to corrosion monitoring. The first is to make a “direct” measurement of the physical properties of the structure itself. The second is to use a “surrogate” material positioned in the service area, which is identical to the material in the structure, and infer the corrosion of the structure from the surrogate material. The third is to monitor the “chemistry” of the solution or gas upstream, downstream or within the service environment and then infer the effects of corrosion on the structure from an empirical or theoretical relationship that relates the measured quantity to the corrosion-induced damage.
The objective of all three corrosion-monitoring methods is to predict the remaining useful life of the structure of interest from an estimate of the corrosion measured or inferred with the monitoring method. In the case of monitoring the structure directly, a simple extrapolation can be made once several time-sequenced measurements have been made. In the case of either monitoring corrosive chemistry or monitoring surrogates, an inference must be made that correlates the corrosion measurement taken to the actual impacts on the structure.
Direct monitoring is a preferred method, but due to access, safety, or cost implications, this approach is not always viable. Direct monitoring may involve visual or photographic inspection of the structure, or physical measurements of the dimensions of the structure, e.g., wall thickness; pit depth, diameter or pit density; or crack depth, width, length or density. The main problem with direct monitoring is the access to the structure is needed and in many instances, access is not possible. Such measurements cannot be practically be made, for example, in radioactive storage containers, or on the walls of underground or the floor of aboveground storage tanks and piping containing petroleum or other hazardous substances and hazardous waste. For these types of applications, surrogate monitoring and chemistry monitoring systems are normally employed.
There are commercially available corrosion monitoring techniques that involve direct monitoring of a surrogate. The surrogate material is typically made of the same material as the structure of interest. The most common surrogate monitoring approach is the direct placement of corrosion coupons in the environment of interest. A corrosion coupon is a piece of material similar, or preferably identical to, the material of interest. The corrosion coupon(s) are placed in similar service conditions and then removed from the service area and evaluated at a later date. These coupon inspections are done periodically and are not linked to a specific level of corrosion. The coupons may be analyzed using destructive metallography. They may be inspected for appearance and/or weighed and compared to the pre-service weight to determine material loss. The use of corrosion coupons, while viewed as a very good method of assessing corrosion, is typically expensive and inconvenient to use. In some instances, the structure needs to be taken out of service to remove the coupons from the service area, which is expensive and may have health and safety implications. As presently used, corrosion coupons do not give any early warning of impending failure until they are retrieved and examined.
Corrosion on the internal wall of oil and gas pipelines occurs when the pipe wall is exposed to water and contaminants in the fluid product or internal gas atmosphere. The nature and extent of the corrosion damage that occurs are functions of the concentration and particular combination of various corrosive constituents within the pipe, as well as the operating conditions of the pipeline. The impact of this contamination is the loss of pipe wall thickness that can result in pipe cracking and rupturing.
Similar degradation issues exist on the exterior surfaces of buried pipelines, where sacrificial coupons may be placed on the surface of the pipe and periodically removed for inspection and analysis. Similarly, other underground concrete building structures, underwater structures and the like are susceptible to corrosion and general metal loss due to surrounding environmental conditions that are monitored with the use of such coupons. Regardless, the current methods of coupon removal and replacement are fraught with measurement errors.
Two common methods of measuring the internal wall integrity are with (1) sacrificial metal “coupons” placed into the pipe's product stream that can be periodically removed and examined to determine the amount of metal loss which is then extrapolated as a representation of the pipe's internal condition, or (2) electrical resistance probes, which are inserted into the pipe's product stream and monitor the rate of corrosion of an exposed sample of metal through the change of resistance of an electrical current applied to the probe. Sample measurements are taken over a period of time with an electrical current measurement meter. Changes in resistance are translated into metal loss that yields the amount of loss over a period of time (typically, mils per year).
Both methods are considered to be relatively inaccurate, yield inconsistent readings, and create handling challenges. Thus, there remains a need within the industry for an improved method and apparatus for monitoring pipeline and storage tank corrosion.